1 Technical Field of the Invention
The present invention relates generally to a gas sensor which may be employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of gas such O2, NOx, or CO, and more particularly to an improved structure of a ceramic heater used in gas sensors and a manufacturing method thereof.
2 Background Art
FIGS. 1(a) and 1(b) show one example of conventional ceramic heaters which is built in an oxygen sensor for use in air-fuel ratio control of automotive internal combustion engines. The ceramic heater 9 serves to heat a sensor element up to an elevated temperature to minimize a variation in measured value.
The ceramic heater 9 consists of a ceramic square rod 10 made of a laminate of heater substrates and a covering substrate and metallic terminals 3 mounted on side surfaces 15 of the rod 10. The metallic terminals 3 connect electrically with leads of a heater-patterned layer in the rod 10 and joined to outer leads 4 through solders 5, respectively.
In manufacturing the ceramic heater 9, green sheets 101 and 102, as shown in FIG. 2(a), whose main component is alumina are first prepared. Next, a conductive paste is applied to the surface of each of the green sheets 101 to form a heater-patterned layer 2 consisting of pairs of a heater element 21 and a lead 22. The two green sheets 101 and the covering green sheet 102 are laid to overlap each other to form a three-layer laminate. The three-layer laminate is cut into several pieces as shown in FIG. 2(b). The metallic terminals 3 are formed on the side surfaces 15 of each piece which communicate electrically with the leads 22 to make an intermediate. Subsequently, the intermediate is baked, after which the outer leads 4 is, as shown in FIG. 2(c), welded to the metallic terminals 3 through the solder 5. Finally, welded portions of the outer leads 4 are, as indicated at numeral 6 in FIG. 1(b), plated with Ni to make the ceramic heater 9.
The above ceramic heater 9 and the manufacturing method thereof, however, have the following drawbacks.
The metallic terminals 3 are, as described above, mounted on the side surfaces 15 of the ceramic heater 9. It is, thus, only possible to attach the metallic terminals 3 to the square rod 10 after the three-layer laminate is cut as shown in FIG. 2(b). In other words, a large number of terminal attachment processes are required in mass-production of ceramic heaters.
In addition, the performance of the ceramic heater 9 is usually inspected after the outer leads 4 are mounted thereon. A large number of individual inspections are also required in the mass-production of ceramic heaters, thus resulting in an increase in manufacturing cost.
Another problem is also encountered in that the ceramic heater 9 is lower in durability than a round rod heater 91 as shown in FIG. 3(a). The results of heat cycle tests show that portions of the ceramic heater 9 welded to the outer leads 4 and the metallic terminals 3 tend to be cracked as compared with the round rod heater 91. This is because the angle xcex2 which each of the metallic terminals 3 of the ceramic heater 9, as shown in FIG. 4, makes with the outer surface of the solder 5 is greater than the angle xcex1 which each of the metallic terminals 3 of the round rod heater 91, as shown in FIG. 3(b), makes with the outer surface of the solder 5. The difference between the angles xcex1 and xcex2 depends upon the geometry of the heaters 9 and 91 and thus is difficult to eliminate. The use of solder which is soft enough to absorb internal stress ensures substantially the same durability of the portions of the rod 10 welded to the leads 4 as that of the round rod heater 91, however, square rod heaters exhibiting higher durability even in use of harder solder is sought.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide an easy-to-manufacture ceramic heater used in gas sensors which has a high durability and a manufacturing method thereof.
According to one aspect of the invention, there is provided a ceramic heater which may be employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of gas such O2, NOx, or CO. The ceramic heater comprises: (a) a ceramic square rod formed with a laminate of a heater substrate on which a heater-patterned layer consisting of a heater element and leads connected to the heater element is formed and a covering substrate covering the heater-patterned layer of the heater substrate; (b) metallic terminals connected electrically to the leads of the heater-patterned layer of the heater substrate, respectively, the metallic terminals being mounted on surfaces of the ceramic square rod opposed to each other in a direction of lamination of the heater substrate and the covering substrate, respectively; and (c) at least one outer lead joined to one of the metallic terminals through a bonding layer.
In the preferred mode of the invention, a second outer lead is further joined to the other metallic terminal through a bonding layer.
The metallic terminals are electrically connected to the leads through holes formed in at least one of the covering substrate and the heater substrate.
Each of the metallic terminals is mounted on an area inside edges of the surface of the ceramic square rod.
The bonding layer occupies an area of a surface of the metallic terminal inside edges of the metallic terminal.
The one of the metallic terminals contains 70 Wt % of W or more. The bonding layer contains 40 to 98 Wt % of Cu and 2 to 20 Wt % of Ni.
The bonding layer may contain 60 Wt % of Au or less.
An Ni-plated layer may be formed on the one of the metallic terminals, having a thickness of 3 xcexcm or less. The outer lead is joined to the Ni-plated layer through the bonding layer.
According to the second aspect of the invention, there is provided a ceramic heater. The ceramic heater comprises: (a) a ceramic square rod formed with a laminate of heater substrates each having formed thereon a heater-patterned layer consisting a heater element and first and second leads connected to the heater element and a covering substrate interposed between the heater substrates; (b) first and second metallic terminals connected electrically to the first and second leads of the heater-patterned layers of the heater substrates, respectively, the metallic terminals being mounted on surfaces of the ceramic square rod opposed to each other in a direction of lamination of the heater substrates and the covering substrate; and (c) outer leads joined to the first and second metallic terminals through bonding layers, respectively.
In the preferred mode of the invention, the first metallic terminal is connected to the first leads of the heater substrates through conductive material-coated holes formed in the covering substrate and one of the heater substrates. The second metallic terminal is connected to the second leads of the heater substrates through conductive material-coated holes formed in the covering substrate and the other heater substrate.
Each of the bonding layers occupies an area of a surface of one of the metallic terminals inside edges of the metallic terminal.
Each of the metallic terminals contains 70 Wt % of W or more. Each of the bonding layers contains 40 to 98 Wt % of Cu and 2 to 20 Wt % of Ni.
Each of the bonding layers contains 60 Wt % of Au or less.
An Ni-plated layer formed on each of the metallic terminals, having a thickness of 3 xcexcm or less. The outer leads are joined to the Ni-plated layers through the bonding layers.
According to the third aspect of the invention, there is provided a method of manufacturing ceramic heaters which comprises the steps of: (a) preparing a first green sheet; (b) preparing a second green sheet; (c) printing a first surface of the second green sheet an array of heater-patterned layers each consisting of a heater element and leads connected to the heater element; (d) printing a second surface of the second green sheet opposite the first surface with an array of metallic terminals; (e) attaching the first green sheet to the second green sheet so as to cover the first surface of the second green sheet to form a laminate; (f) baking the laminate to form a ceramic board; (g) joining outer leads to the metallic terminals through bonding layers, respectively; and (h) cutting the ceramic board into a plurality of square rods constituting units of the ceramic heaters.
In the preferred mode of the invention, a step is further provided which forms through holes in the first green sheet for electrical connections of the leads of the heater-patterned layers and the metallic terminals.
A step is further provided which forms grooves in a surface of the ceramic board between adjacent two of the units of the ceramic heaters to be cut by the cutting step.
According to the fourth aspect of the invention, there is provided a method of manufacturing ceramic heaters which comprises the steps of: (a) preparing a first green sheet; (b) preparing second green sheets; (c) printing a first surface of each of the second green sheets an array of heater-patterned layers each consisting of a heater element and leads connected to the heater element; (d) printing a second surface of each of the second green sheets opposite the first surface with an array of metallic terminals; (e) interposing the first green sheet between the second green sheets so as to cover the first surfaces of the second green sheets to form a laminate; (f) baking the laminate to form a ceramic board; (g) joining outer leads to the metallic terminals formed on at least one of the second green sheets through bonding layers, respectively; and (h) cutting the ceramic board into a plurality of square rods constituting units of the ceramic heaters.
According to the fifth aspect of the invention, there is provided a gas sensor which comprises: (a) a gas sensing element having a gas-exposed portion, the gas sensing element having formed therein a chamber; (b) a ceramic heater disposed within the chamber of the gas sensing element to heat the gas sensing element; (c) a first cylindrical holder fitted in the chamber of the gas sensing element, the first holder including a heater holding portion for holding the ceramic heater and a sensor contact in contact with an inner wall of the gas sensing element, the sensor contact having a sensor signal output terminal; (d) a second cylindrical holder mounted on an outer wall of the gas sensing element, having a sensor signal output terminal; and (e) a slit formed in the first holder to define a C-shaped cross section, the slit being located 90xc2x0xc2x120xc2x0 apart from the sensor signal output terminal of the first cylindrical holder. The ceramic heater includes, (a) a ceramic square rod formed with a laminate of a heater substrate on which a heater-patterned layer consisting of a heater element and leads connected to the heater element is formed and a covering substrate covering the heater-patterned layer of the heater substrate, (b) metallic terminals connected electrically to the leads of the heater-patterned layer of the heater substrate, respectively, the metallic terminals being mounted on surfaces of the ceramic square rod opposed to each other in a direction of lamination of the heater substrate and the covering substrate, respectively, and (c) at least one outer lead joined to one of the metallic terminals through a bonding layer.
In the preferred mode of the invention, the sensor signal output terminal of the first cylindrical holder is located 180xc2x0 apart from the sensor signal output terminal of the second cylindrical holder.
The slit is located 90xc2x0 apart from the sensor signal output terminal of the first cylindrical holder.